Functional diene block elastomer with a low pi and improved cold flow, and rubber composition containing same

ABSTRACT

The invention relates to a functionalized diene elastomer composed a) of a specific block copolymer functionalized, at the chain end or in the middle of the chain, by a tin functional group, b) of a specific block copolymer star-branched by tin and c) of a content of less than 15% by weight, with respect to the total weight of the functionalized diene elastomer, of a specific non-tin-functional block copolymer.

BACKGROUND

1. Field

The present invention relates to a functionalized diene elastomercomposed of a specific functionalized block copolymer, of a specificstar-branched block copolymer and of a content of less than 15% byweight of a specific non-tin-functional block copolymer.

2. Description of Related Art

The reduction in the hysteresis of the mixtures is an ongoing objectiveof the tire industry in order to limit the consumption of petrol and tothus protect the environment. This reduction in hysteresis must,however, be carried out while keeping intact, indeed even whileimproving, the processability of the mixtures.

Many solutions have already been experimented with in order to achievethe objective of fall in hysteresis. In particular, thefunctionalization of the polymers by a functional group which interactswith the reinforcing filler has appeared as an advantageous route.

Functional groups which interact with the filler have thus been attachedat the chain end at the start or end of polymerization by means offunctional initiators or functionalization agents. By way of example,4,4′-bis(diethylamino)benzophenone, also known as DEAB, or otheraminated functional groups which interact with carbon black have beenadded at the end of polymerization, as described in the patent documentsFR 2 526 030 and U.S. Pat. No. 4,848,511. The polymers coupled by orstar-branched by tin comprise functional groups capable of interactingwith carbon black which are introduced at the end of polymerization.Mention may be made, by way of example, of the European patent documentEP 0 709 235. Functional groups which interact with silica have alsobeen attached at the chain end at the end of polymerization, such asfunctional groups comprising a silanol group which are disclosed in thepatent document FR 2 740 778 or functional groups comprisingalkoxysilane or aryloxysilane groups which are described in the documentU.S. Pat. No. 5,066,721. The majority of these solutions, both for theblack and for the silica, genuinely result in a limitation on hysteresisof the corresponding compositions but concomitantly in a greaterdifficulty in processing these same compositions.

Patent EP 1 278 789 describes a copolymer comprising n blocks (n=2 or 3)intended to form an elastomer matrix of a crosslinkable rubbercomposition, each of the said blocks comprising an essentiallyunsaturated diene elastomer and one or each of the said blocks forming achain end of the said copolymer being composed of a polyisoprene. Thenumber-average molecular weight of the polyisoprene block is between 2500 and 20 000 g/mol and the number-average molecular weight of theblock of the copolymer which is other than the said polyisoprene blockis substantially between 80 000 g/mol and 350 000 g/mol. The use of thesaid block copolymer makes it possible to significantly optimise theresults of reduction in hysteresis and processability for the saidrubber composition in which it is present.

It is also known that polymers having narrow molecular distributionsconfer a reduced hysteresis on the rubber compositions in which they arepresent. In particular, linear functional diene elastomers with narrowmolecular distributions before functionalization exhibit improvedhysteresis properties. However, these elastomers exhibit an increasedcold flow in comparison with the same elastomers exhibiting a broadmolecular distribution before functionalization, which is damaging forthe storage and transportation of the elastomers.

Many solutions exist in order to reduce the cold flow. The workNouvelles Recherches dans le Domaine des Composés Macromoléculaires [NewResearch in the Field of Macromolecular Compounds], editor(s) CeausescuE, Oxford, Pergamon Press Ltd., 1984, pp. 343-56, 72, mentions methodsfor reducing cold flow, such as the increase in the molecular weight,the star-branching or the degree of branching. However, the increase inthe molecular weight is highly damaging to the use of the mixtures whilethe branching is accompanied by an increase in the hysteresis of themixtures. Furthermore, surprisingly, the polymers solely star-branchedby tin (3 or 4 branches) exhibit greater hysteresis in comparison withthe polymers coupled by tin.

It is also known that, in order to solve the problem of cold flow, thepolydispersity index of the elastomer can be increased. However, this isnot without effect on the properties of the rubber compositions in whichit is present, in particular on the hysteresis or the processability ofthe mixtures, for example.

SUMMARY

There thus exists a need to provide an elastomer which confers, on areinforced rubber composition, good properties of hysteresis and ofprocessing for the purpose of a tire application, while exhibiting areduced cold flow from the perspective of better behaviour duringstorage and transportation of the rubber.

The Applicant Company has discovered, surprisingly, that afunctionalized diene elastomer composed of a block copolymerfunctionalized at the chain end or in the middle of the chain by a tinfunctional group, exhibiting a narrow molecular weight distributionbefore functionalization or coupling, and of a block copolymerstar-branched with a tin-comprising compound, exhibiting a narrowmolecular weight distribution before star-branching and comprising lessthan 15% by weight, with respect to the total weight of thefunctionalized diene elastomer, of non-tin-functional block copolymers,confers, on a rubber composition in which it is present, rubberproperties, in particular hysteresis and processing properties, whichare entirely acceptable for use in tires, while exhibiting asignificantly improved resistance to cold flow.

A subject-matter of the invention is thus a functionalized dieneelastomer composed:

a) of a block copolymer functionalized, at the chain end or in themiddle of the chain, by a tin functional group and corresponding to thefollowing formula:

[A-B_(n)XB-A]_(m)

where n and m are integers of greater than or equal to 0 and such thatn+m=1 or 2,

b) of a block copolymer star-branched by tin and corresponding to thefollowing formula:

[A-B_(o)YB-A]_(p)

where o and p are integers of greater than or equal to 0 and such thato+p≧3 and o+p≦6,

c) of a content of less than 15% by weight, with respect to the totalweight of the functionalized diene elastomer, of a non-tin-functionalblock copolymer corresponding to the following formula:

[A-B]

where:

-   -   all the A blocks are composed of a polyisoprene or all the A        blocks are composed of a polybutadiene,    -   the B blocks are composed of a diene elastomer, the molar        content of units resulting from conjugated dienes of which is        greater than 15%, the B blocks being identical to one another,    -   X and Y independently represent a tin-comprising group,    -   the number-average molecular weight Mn1 of each A block varies        from 2 500 to 20 000 g/mol,    -   the number-average molecular weight Mn2 of each B block varies        from 80 000 to 350 000 g/mol,    -   the content of 1,2 linkages in each A block is between 1 and 20%        in the case where A is a polybutadiene block,    -   the content of 3,4 linkages in each A block is between 1 and 25%        in the case where A is a polyisoprene block,    -   the A-B copolymer exhibits a monomodal distribution of molecular        weights before optional functionalization or optional        star-branching and a polydispersity index before optional        functionalization or optional star-branching of less than or        equal to 1.3.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The term “polydispersity index” is understood to mean, within themeaning of the invention, the weight-average molecularweight/number-average molecular weight ratio.

Preferably, the functionalized diene elastomer according to theinvention comprises from 5% to 45% by weight, preferably from 10% to 30%by weight, with respect to the total weight of the functionalized dieneelastomer, of the said block copolymer star-branched by tin b).

Preferably, the functionalized diene elastomer according to theinvention comprises a content strictly of greater than 0% by weight andof less than 10% by weight, with respect to the total weight of thefunctionalized diene elastomer, of the said non-tin-functional blockcopolymer c). Preferably, the functionalized diene elastomer accordingto the invention comprises a content of less than 5% by weight, withrespect to the total weight of the functionalized diene elastomer, ofthe said non-tin-functional block copolymer c).

In the present description, the term “functionalized diene elastomer” isunderstood to mean a diene elastomer which comprises a group comprisingone or more heteroatoms.

This group can be located at the end of the chain formed by the B blockother than the polyisoprene or polybutadiene block A (m+n=1, m and nbeing defined as above). It will then be said that the diene elastomeris functionalized at the end of the said chain. It is generally anelastomer obtained by reaction of a living elastomer with afunctionalization agent, that is to say any at least monofunctionalmolecule, the functional group being any type of chemical group known bya person skilled in the art to react with a living chain end.

This group can be located in the linear main elastomer chain between twoB blocks other than the polyisoprene or polybutadiene blocks A (m+n=2, mand n being defined as above). It will then be said that the dieneelastomer is coupled or alternatively functionalized in the middle ofthe said chain, in contrast to the position “at the end of the chain”,although the group is not located precisely at the middle of the saidelastomer chain. It is generally an elastomer obtained by reaction of aliving elastomer with a coupling agent, that is to say any at leastdifunctional molecule, the functional group being any type of chemicalgroup known by a person skilled in the art to react with a living chainend.

This group can be central, to which o+p elastomer or branched chains A-B(o, p and A-B being defined as above) are bonded, forming astar-branched structure of the elastomer. It will then be said that thediene elastomer is star-branched. It is generally an elastomer obtainedby reaction of a living elastomer with a star-branching agent, that isto say any polyfunctional molecule, the functional group being any typeof chemical group known by a person skilled in the art to react with aliving chain end.

As explained above, the block copolymer a) is functionalized at the endor in the middle of the chaine formed by a tin functional group. Thefunctionalization can be obtained with a monohalotin functionalizationagent or a dihalotin coupling agent which can correspond to the generalformula R_(4-x)SnX⁰ _(x), where x represents an integer having the value1 or 2, R represents an alkyl, cycloalkyl, aryl, alkaryl or vinylradical having from 1 to 12 carbon atoms, preferably a butyl, and X⁰ isa halogen atom, preferably chlorine. Mention may be made, as preferredfunctionalization agent, of tributyltin monochloride or dibutyltindichloride. In the same way, the functionalization can be obtained witha tin-derived functionalization agent which can correspond to thegeneral formula (X¹ _(y)R¹ _(3-y)Sn)—O—(SnR¹ _(3-z)X¹ _(z)) or (X¹_(y)R¹ _(3-y)Sn)—O—(CH₂)_(e)—O—(SnR¹ _(3-z)X¹ _(z)), where y and zrepresent integers between 0 and 2 and y+z is equal to 1 or 2, R¹represents an alkyl, cycloalkyl, aryl, alkaryl or vinyl radical havingfrom 1 to 12 carbon atoms, preferably a butyl, X¹ is a halogen atom,preferably chlorine, and e represents an integer from 1 to 20,preferably 4.

According to the invention, the block copolymer b) is star-branched.Preferably, the diene elastomer b) is star-branched by a tin functionalgroup. The star-branching can be obtained with a tri- or tetrahalotinstar-branching agent which can correspond to the formula R² _(q)SnX²_(4-q), where q represents an integer having the value 0 or 1, R²represents an alkyl, cycloalkyl, aryl, alkaryl or vinyl radical havingfrom 1 to 12 carbon atoms, preferably a butyl, and X² is a halogen atom,preferably chlorine. Mention may be made, as preferred star-branchingagent, of butyltin trichloride or tin tetrachloride. In the same way,the star-branching can be obtained with a tin-derived functionalizationagent which can correspond to the general formula (X³ _(k)R³_(3-k)Sn)—O—(SnR³ ₃₋₁X³ ₁) or (X³ _(k)R³ _(3-k)Sn)—O—(CH₂)_(f)—O—(SnR³₃₋₁X³ ₁), where k and 1 represent integers between 0 and 3 and k+1integers between 3 and 6, R³ represents an alkyl, cycloalkyl, aryl,alkaryl or vinyl radical having from 1 to 12 carbon atoms, preferably abutyl, X³ is a halogen atom, preferably chlorine, and f represents aninteger having a value from 1 to 20, preferably 4.

According to a preferred embodiment, the block copolymer a) is acopolymer functionalized by a tin functional group in the middle of thechain.

According to another preferred embodiment, the block copolymer b) is acopolymer star-branched by tin having 4 branches.

According to another preferred embodiment, the block copolymer a) is acopolymer functionalized by a tin functional group in the middle of thechain and the block copolymer b) is a copolymer star-branched by tinhaving 4 branches.

Preferably, the ratio of the number-average molecular weight Mn1 of eachend polybutadiene or polyisoprene block A to the number-averagemolecular weight Mn2 of each of the B blocks varies from 5 to 20%.

As explained above, preferably, the block copolymer star-branched by tinrepresents from 5 to 45% by weight, preferably from 10 to 30% by weight,of the total weight of the functionalized diene elastomer.

Essentially unsaturated diene elastomer (i.e., the molar content ofunits resulting from conjugated dienes of which is greater than 15%)capable of being employed in order to obtain the said B block other thanthe polybutadiene or polyisoprene block or blocks, themselvescorresponding to this definition, is understood to mean any homopolymerobtained by polymerization of a conjugated diene monomer having from 4to 12 carbon atoms, or any block, random, sequential or microsequentialcopolymer obtained by copolymerization of one or more conjugated dieneswith one another or with one or more vinylaromatic compounds having from8 to 20 carbon atoms.

The following are suitable in particular as conjugated dienes:1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C₁-C₅alkyl)-1,3-butadienes, such as 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene,1,3-pentadiene or 2,4-hexadiene. The following, for example, aresuitable as vinylaromatic compounds: styrene, ortho-, meta- orpara-methylstyrene, the “vinyltoluene” commercial mixture,para-(tert-butyl)styrene, methoxystyrenes, chloro styrenes,vinylmesitylene, divinylbenzene or vinylnaphthalene.

Preferably, the B block or blocks are chosen from copolymers of styreneand butadiene, copolymers of styrene and isoprene, copolymers ofbutadiene and isoprene, styrene/butadiene/isoprene terpolymers,polyisoprene when the neighbouring A block is a polybutadiene andpolybutadiene when the neighbouring A block is a polyisoprene.

The following are suitable: polybutadienes, in particular those having acontent (mol %) of 1,2- units of between 4% and 80%, polyisoprenes,butadiene/styrene copolymers and in particular those having a Tg (glasstransition temperature Tg, measured according to ASTM D3418) of between0° C. and −70° C. and more particularly between −10° C. and −60° C., astyrene content of between 5% and 60% by weight and more particularlybetween 20% and 50%, a content (mol %) of 1,2- bonds of the butadienepart of between 4% and 75% and a content (mol %) of trans-1,4- bonds ofbetween 10% and 80%, butadiene/isoprene copolymers, in particular thosehaving an isoprene content of between 5% and 90% by weight and a Tg of−40° C. to −80° C., or isoprene/styrene copolymers, in particular thosehaving a styrene content of between 5% and 50% by weight and a Tg ofbetween 5° C. and −55° C.

In the case of butadiene/styrene/isoprene copolymers, those having astyrene content between 5% and 50% by weight and more particularly, ofbetween 10% and 40%, an isoprene content of between 15% and 60% byweight and more particularly between 20% and 50%, a butadiene content ofbetween 5% and 50% by weight and more particularly of between 20% and40%, a content (mol %) of 1,2- units of the butadiene part of between 4%and 85%, a content (mol %) of trans-1,4- units of the butadiene part ofbetween 6% and 80%, a content (mol %) of 1,2- plus 3,4- units of theisoprene part of between 5% and 70% and a content (mol %) of trans-1,4-units of the isoprene part of between 10% and 50%, and more generallyany butadiene/styrene/isoprene copolymer having a Tg of between −5° C.and −70° C., are suitable in particular.

Preferably, the B block or blocks are chosen from copolymers of styreneand butadiene, copolymers of styrene and isoprene, polyisoprene when theneighbouring A block is a polybutadiene and polybutadiene when theneighbouring A block is a polyisoprene. More preferably, the B block orblocks are chosen from copolymers of styrene and butadiene.

According to a particularly preferred embodiment, A is polyisoprene andB is a copolymer of styrene and butadiene, the block copolymer a) beingfunctionalized by the functionalization agent Bu₂SnCl₂ and the blockcopolymer b) being star-branched by the star-branching agent 5 nCl₄.

According to another particularly preferred embodiment, A ispolybutadiene and B is a copolymer of styrene and butadiene, the blockcopolymer a) being functionalized by the functionalization agentdibutyltin dichloride (Bu₂SnCl₂) and the block copolymer b) beingstar-branched by the star-branching agent tin tetrachloride (SnCl₄).

The polymerization of diene monomers is initiated by an initiator. Usemay be made, as polymerization initiator, of any known monofunctionalanionic initiator. However, an initiator comprising an alkali metal,such as lithium, is preferably used.

Those comprising a carbon-lithium bond are suitable in particular asorganolithium initiators. Use will preferably be made of a hydrocarbonorganolithium initiator not comprising a heteroatom. Representativecompounds are aliphatic organolithium compounds, such as ethyllithium,n-butyllithium (n-BuLi) and isobutyllithium.

The polymerization is, as known per se, preferably carried out in thepresence of an inert solvent which can, for example, be an aliphatic oralicyclic hydrocarbon, such as pentane, hexane, heptane, isooctane orcyclohexane, or an aromatic hydrocarbon, such as benzene, toluene orxylene.

The polymerization can be carried out continuously or batchwise,preferably batchwise. The polymerization is generally carried out at atemperature of between 20° C. and 120° C. and preferably in the vicinityof 30° C. to 90° C. It is, of course, also possible to add, at the endof polymerization, a transmetallation agent for modifying the reactivityof the living chain end.

The living diene elastomer resulting from the polymerization issubsequently functionalized in order to prepare the functionalized dieneelastomer according to the invention.

According to one method of preparation of the functionalized dieneelastomer according to the invention, the block copolymer at the livingchain end can be prepared in different stages:

-   -   the preparation of the living polyisoprene or polybutadiene        block, and    -   the preparation of the essentially unsaturated diene elastomer        carried out in order to obtain the said block other than the        polybutadiene or polyisoprene block.

According to this method of preparation, the polymerization of isopreneor butadiene diene monomers is initiated by the said organolithiuminitiators in order to obtain a living polyisoprene or polybutadienediene homopolymer. The living diene homopolymer thus obtained issubsequently used as initiator in the preparation of the diene elastomerin order to obtain a living block copolymer.

The well-informed reader would understand that, during the preparationstages, appropriate processing conditions have to be deployed in orderto limit the formation of dead or deactivated polyisoprene orpolybutadiene diene homopolymer which thus generates chains having lowmolecular weights. An amount of greater than 1% by weight of thesepolyisoprene or polybutadiene chains would be damaging to the propertiesof the functionalized diene elastomer according to the invention.

The living block copolymer resulting from this method of preparation issubsequently functionalized in order to prepare the functionalized dieneelastomer according to the invention.

According to a first alternative form of the preparation of thefunctionalized diene elastomer according to the invention, the blockcopolymer functionalized at the end of or in the middle of the chain bya tin functional group a) and the star-branched block copolymer b) aremixed in the appropriate proportions.

The block copolymer a) functionalized at the end of or in the middle ofthe said chain by a tin functional group can be obtained, in a way knownper se, by reaction of a tin derivative with the living diene elastomerresulting from the polymerization.

The star-branched block copolymer b) can be obtained, in a way known perse, by reaction of a tin-comprising star-branching agent with the livingdiene elastomer resulting from the polymerization.

The mixing of the two elastomers can be carried out in an inert solvent,for example an aliphatic or alicyclic hydrocarbon, such as pentane,hexane, heptane, isooctane or cyclohexane, or an aromatic hydrocarbon,such as benzene, toluene or xylene, which can be the same as thepolymerization solvent. The mixing is then carried out at a temperaturepreferably of between 20° C. and 120° C., preferably in the vicinity of30° C. to 90° C.

According to a second alternative form of the preparation of thefunctionalized diene elastomer according to the invention, the livingdiene elastomer resulting from the polymerization stage is subjected tothe reaction of a functionalization agent and to the reaction of astar-branching agent.

Thus, for example, the functionalization of the living diene elastomerresulting from the polymerization stage can be carried out at atemperature varying from 30° C. to 120° C., in the presence, to beginwith, of an appropriate amount of a star-branching agent in order tostar-branch preferably from 5% to 45% by weight of the living elastomer.Then, subsequently, the remaining living chains of the diene elastomerobtained after the first stage are functionalized by addition of a tinfunctionalization agent capable of introducing a tin functional group atthe end of the chain or in the middle of the chain. Thefunctionalization reaction of the diene elastomer is subsequentlystopped by the deactivation of the remaining living chains.

The well-informed reader would understand that, during the stages ofpreparation of the functionalized block copolymers a) and b),appropriate processing conditions have to be deployed in order to limitthe formation of the non-tin-functionalized copolymer c).

A further subject-matter of the invention is a reinforced rubbercomposition based on at least one reinforcing filler and on an elastomermatrix comprising at least one functionalized diene elastomer accordingto the invention.

The composition can comprise from 1 to 100 phr of functionalized dieneelastomer according to invention.

The composition according to the invention can also comprise at leastone diene elastomer other than the said functionalized diene elastomeraccording to the invention This or these diene elastomers other than thefunctionalized diene elastomer according to the invention can be chosenfrom the diene elastomers conventionally used in tires, such as naturalrubber or a synthetic elastomer, or also another functionalized orstar-branched elastomer.

Use may be made of any type of reinforcing filler known for itsabilities to reinforce a rubber composition which can be used for themanufacture of tires, for example a reinforcing organic filler, such ascarbon black, a reinforcing inorganic filler, such as silica, or a blendof these two types of filler, in particular a blend of carbon black andsilica.

All carbon blacks, in particular blacks of the HAF, ISAF or SAF type,conventionally used in tires (“tire-grade” blacks) are suitable ascarbon blacks. Mention will more particularly be made, among the latter,of the reinforcing carbon blacks of the 100, 200 or 300 series (ASTMgrades), such as, for example, the N115, N134, N234, N326, N330, N339,N347 or N375 blacks.

Use may also be made, according to the applications targeted, of blacksof higher series FF, FEF, GPF or SRF, for example the N660, N683 or N772blacks. The carbon blacks might, for example, be already incorporated inthe isoprene elastomer in the form of a masterbatch (see, for example,Applications WO 97/36724 or WO 99/16600).

Mention may be made, as examples of organic fillers other than carbonblacks, of functionalized polyvinylaromatic organic fillers, such asdescribed in Applications WO-A-2006/069792 and WO-A-2006/069793.

The term “reinforcing inorganic filler” should be understood, in thepresent patent application, by definition, as meaning any inorganic ormineral filler, whatever its colour and its origin (natural orsynthetic), also known as “white filler”, “clear filler” or indeed even“non-black filler”, in contrast to carbon black, capable of reinforcingby itself alone, without means other than an intermediate couplingagent, a rubber composition intended for the manufacture of tires, inother words capable of replacing, in its reinforcing role, aconventional tire-grade carbon black; such a filler is generallycharacterized, in a known way, by the presence of hydroxyl (—OH) groupsat its surface.

The physical state under which the reinforcing inorganic filler isprovided is not important, whether it is in the form of a powder, ofmicrobeads, of granules, of beads or any other appropriate densifiedform. Of course, the term “reinforcing inorganic filler” is alsounderstood to mean mixtures of different reinforcing inorganic fillers,in particular of highly dispersible siliceous and/or aluminous fillersas described below.

Mineral fillers of the siliceous type, in particular silica (SiO₂), orof the aluminous type, in particular alumina (Al₂O₃), are suitable inparticular as reinforcing inorganic fillers. The silica used can be anyreinforcing silica known to a person skilled in the art, in particularany precipitated or fumed silica exhibiting a BET specific surface and aCTAB specific surface both of less than 450 m²/g, preferably from 30 to400 m²/g. Mention will be made, as highly dispersible precipitatedsilicas (“HDSs”), for example, of the Ultrasil 7000 and Ultrasil 7005silicas from Degussa, the Zeosil 1165 MP, 1135 MP and 1115 MP silicasfrom Rhodia, the Hi-Sil EZ150G silica from PPG, the Zeopol 8715, 8745and 8755 silicas from Huber or the silicas with a high specific surfaceas described in Application WO 03/16837.

When the composition according to the invention is intended for tiretreads having a low rolling resistance, the reinforcing inorganic fillerused, in particular if it is silica, preferably has a BET specificsurface of between 45 and 400 m²/g, more preferably of between 60 and300 m²/g.

Preferably, the content of reinforcing filler in the composition isbetween 30 and 150 phr, more preferably between 50 and 120 phr. Theoptimum is different according to the specific applications targeted:the expected level of reinforcement with regard to a bicycle tire, forexample, is, of course, lower than that required with regard to a tirecapable of running at high speed in a sustained manner, for example amotorcycle tire, a tire for a passenger vehicle or a tire for a utilityvehicle, such as a heavy-duty vehicle.

According to one embodiment, the reinforcing filler predominantlycomprises silica, the content of carbon black present in the compositionpreferably being between 2 and 20 phr.

According to another embodiment of the invention, the reinforcing fillerpredominantly comprises carbon black.

Use is made, in a known way, in order to couple the reinforcinginorganic filler to the diene elastomer, of an at least bifunctionalcoupling agent (or bonding agent) intended to provide a satisfactoryconnection, of chemical and/or physical nature, between the inorganicfiller (surface of its particles) and the diene elastomer, in particularbifunctional organosilanes or polyorganosiloxanes.

Use is made in particular of silane polysulphides, referred to as“symmetrical” or “unsymmetrical” according to their specific structure,such as described, for example, in Applications WO 03/002648 (or US2005/016651) and WO 03/002649 (or US 2005/016650).

Suitable in particular, without the definition below being limiting, aresilane polysulphides known as “symetrical”, corresponding to thefollowing general formula (III):

Z-A-S_(x)-A-Z,  (III)

in which:

-   -   x is an integer from 2 to 8 (preferably from 2 to 5);    -   A is a divalent hydrocarbon radical (preferably, C₁-C₁₈ alkylene        groups or C₆-C₁₂ arylene groups, more particularly C₁-C₁₀, in        particular C₁-C₄, alkylenes, especially propylene);    -   Z corresponds to one of the formulae below:

in which:

-   -   the R¹ radicals, which are substituted or unsubstituted and        identical to or different from one another, represent a C₁-C₁₈        alkyl group, a C₅-C₁₈ cycloalkyl group or a C₆-C₁₈ aryl group        (preferably C₁-C₆ alkyl, cyclohexyl or phenyl groups, in        particular C₁-C₄ alkyl groups, more particularly methyl and/or        ethyl);    -   the R² radicals, which are substituted or unsubstituted and        identical to or different from one another, represent a C₁-C₁₈        alkoxy group or a C₅-C₁₈ cycloalkoxyl group (preferably a group        chosen from C₁-C₈ alkoxyls and C₅-C₈ cycloalkoxyls, more        preferably still a group chosen from C₁-C₄ alkoxyls, in        particular methoxyl and ethoxyl).

In the case of a mixture of alkoxysilane polysulphides corresponding tothe above formula (III), in particular the standard commerciallyavailable mixtures, the mean value of the “x” symbols is a fractionalnumber preferably between 2 and 5, more preferably close to 4. However,the invention can also advantageously be carried out, for example, withalkoxysilane disulphides (x=2).

Mention will more particularly be made, as examples of silanepolysulphides, of bis((C₁-C₄)alkoxyl(C₁-C₄)alkylsilyl(C₁-C₄)alkyl)polysulphides (in particular disulphides, trisulphides ortetrasulphides), such as, for example, bis(3-trimethoxysilylpropyl) orbis(3-triethoxysilylpropyl)polysulphides. Use is in particular made,among these compounds, of bis(3-triethoxysilylpropyl)tetrasulphide,abbreviated to TESPT, of formula [(C₂H₅O)₃Si(CH₂)₃S₂]₂, orbis(triethoxysilylpropyl)disulphide, abbreviated to TESPD, of formula[(C₂H₅O)₃Si(CH₂)₃S]₂. Mention will also be made, as preferred examples,of bis(mono(C₁-C₄)alkoxydi(C₁-C₄)alkylsilylpropyl) polysulphides (inparticular disulphides, trisulphides or tetrasulphides), moreparticularly bis(monoethoxydimethylsilylpropyl) tetrasulphide, such asdescribed in the Patent Application WO 02/083782 (or US 2004/132880).

Mention will in particular be made, as coupling agent other than analkoxysilane polysulphide, of bifunctional POSs (polyorganosiloxanes) orelse of hydroxysilane polysulphides (R²=OH in the above formula III),such as described in Patent Applications WO 02/30939 (or U.S. Pat. No.6,774,255) and WO 02/31041 (or US 2004/051210), or else of silanes orPOSs carrying azodicarbonyl functional groups, such as described, forexample, in Patent Applications WO 2006/125532, WO 2006/125533, WO2006/125534 and WO 2009/062733.

In the rubber composition, the content of coupling agent is preferablybetween 0.5 and 12 phr, more preferably between 3 and 8 phr.

Typically, the content of coupling agent represents from 0.5% to 15% byweight, with respect to the amount of inorganic filler.

A person skilled in the art will understand that a reinforcing filler ofanother nature, in particular organic nature, might be used as fillerequivalent to the reinforcing inorganic filler described in the presentsection, provided that this reinforcing filler is covered with aninorganic layer, such as silica, or else comprises, at its surface,functional sites, in particular hydroxyls, requiring the use of acoupling agent in order to form the connection between the filler andthe elastomer.

The composition according to the invention can also comprise a chemicalcrosslinking agent.

The chemical crosslinking makes possible the formation of covalent bondsbetween the elastomer chains. The chemical crosslinking can be carriedout using a vulcanization system or else using peroxide compounds.

The vulcanization system proper is based on sulphur (or on asulphur-donating agent) and on a primary vulcanization accelerator.Additional to this base vulcanization system are various known secondaryvulcanization accelerators or vulcanization activators, such as zincoxide, stearic acid or equivalent compounds, or guanidine derivatives(in particular diphenylguanidine), incorporated during the firstnon-productive phase and/or during the productive phase, as describedsubsequently.

The sulphur is used at a preferred content of between 0.5 and 12 phr, inparticular between 1 and 10 phr. The primary vulcanization acceleratoris used at a preferred content of between 0.5 and 10 phr, morepreferably of between 0.5 and 5.0 phr.

Use may be made, as (primary or secondary) accelerator, of any compoundcapable of acting as accelerator for the vulcanization of dieneelastomers in the presence of sulphur, in particular accelerators of thethiazole type, and also their derivatives, and accelerators of thiuramand zinc dithiocarbamate types. These accelerators are, for example,chosen from the group consisting of 2-mercaptobenzothiazyl disulphide(abbreviated to “MBTS”), tetrabenzylthiuram disulphide (“TBZTD”),N-cyclohexyl-2-benzothiazolesulphenamide (“CBS”),N,N-dicyclohexyl-2-benzothiazolesulphenamide (“DCBS”),N-(tert-butyl)-2-benzothiazolesulphenamide (“TBBS”),N-(tert-butyl)-2-benzothiazolesulphenimide (“TBSI”), zincdibenzyldithiocarbamate (“ZBEC”) and the mixtures of these compounds.

Preferably, use is made of a primary accelerator of the sulphenamidetype.

When the chemical crosslinking is carried out using one or more peroxidecompounds, the said peroxide compound or compounds represent from 0.01to 10 phr.

Mention may be made, as peroxide compounds which can be used as chemicalcrosslinking system, of acyl peroxides, for example benzoyl peroxide orp-chlorobenzoyl peroxide, ketone peroxides, for example methyl ethylketone peroxide, peroxyesters, for example t-butyl peroxyacetate,t-butyl peroxybenzoate and t-butyl peroxyphthalate, alkyl peroxides, forexample dicumyl peroxide, di(t-butyl)peroxybenzoate and1,3-bis(t-butylperoxyisopropyl)benzene, or hydroperoxides, for examplet-butyl hydroperoxide.

The rubber composition according to the invention can also comprise allor a portion of the usual additives generally used in elastomercompositions intended for the manufacture of tires, in particular oftreads, such as, for example, plasticizers or extending oils, whetherthe latter are of aromatic or non-aromatic nature, pigments, protectionagents, such as antiozone waxes (such as Cire Ozone C32 ST), chemicalantiozonants or antioxidants (such as 6-PPD), antifatigue agents,reinforcing resins, methylene acceptors (for example, phenolic novolakresin) or methylene donors (for example, HMT or H3M), as described, forexample, in Application WO 02/10269, or adhesion promoters (cobaltsalts, for example).

Preferably, the composition according to the invention comprises, aspreferred non-aromatic or very weakly aromatic plasticizing agent, atleast one compound chosen from the group consisting of naphthenic oils,paraffinic oils, MES oils, TDAE oils, glycerol esters (in particulartrioleates), plasticizing hydrocarbon resins exhibiting a high Tgpreferably of greater than 30° C., and mixtures of such compounds.

The composition according to the invention can also comprise, inaddition to the coupling agents, activators of the coupling of thereinforcing inorganic filler or more generally processing aids capable,in a known way, by virtue of an improvement in the dispersion of theinorganic filler in the rubber matrix and of a lowering in the viscosityof the compositions, of improving their ease of processing in the rawstate, these processing aids being, for example, hydrolysable silanes,such as alkylalkoxysilanes (in particular alkyltriethoxysilanes),polyols, polyethers (for example, polyethylene glycols), primary,secondary or tertiary amines (for example, trialkanolamines),hydroxylated or hydrolysable POSs, for exampleα,ω-dihydroxypolyorganosiloxanes (in particularα,ω-dihydroxypolydimethylsiloxanes), or fatty acids, such as, forexample, stearic acid.

The rubber composition according to the invention is manufactured inappropriate mixers, using two successive phases of preparation accordingto a general procedure well known to those skilled in the art: a firstphase of thermomechanical working or kneading (sometimes referred to as“non-productive” phase) at high temperature, up to a maximum temperatureof between 130° C. and 200° C., preferably between 145° C. and 185° C.,followed by a second phase of mechanical working (sometimes referred toas “productive” phase) at lower temperature, typically below 120° C.,for example between 60° C. and 100° C., during which finishing phase thechemical crosslinking agent is incorporated.

According to a preferred embodiment of the invention, all the baseconstituents of the composition included in the tire of the invention,with the exception of the chemical crosslinking agent, namely inparticular the reinforcing filler or fillers and the coupling agent, ifappropriate, are intimately incorporated, by kneading, in thefunctionalized diene elastomer and in the other diene elastomers, ifappropriate, during the first “non-productive” phase, that is to saythat at least these various base constituents are introduced into themixer and are thermomechanically kneaded, in one or more stages, untilthe maximum temperature of between 130° C. and 200° C., preferably ofbetween 145° C. and 185° C., is reached.

By way of example, the first (non-productive) phase is carried out in asingle thermomechanical stage during which all the necessaryconstituents, the optional supplementary processing aids and variousother additives, with the exception of the chemical crosslinking agent,are introduced into an appropriate mixer, such as an ordinary internalmixer. The total duration of the kneading, in this non-productive phase,is preferably between 1 and 15 min. After cooling the mixture thusobtained during the first non-productive phase, the chemicalcrosslinking agent is then incorporated at low temperature, generally inan external mixer, such as an open mill; everything is then mixed(productive phase) for a few minutes, for example between 2 and 15 min.

The final composition thus obtained is subsequently calendered, forexample in the form of a sheet or plaque, in particular for laboratorycharacterization, or else extruded in the form of a rubber profiledelement which can be used, for example, as a tire tread for a passengervehicle.

A further subject-matter of the invention is a semi-finished articlemade of rubber for a tire, comprising the crosslinked or crosslinkablerubber composition according to the invention. Preferably, the saidarticle is a tread.

A final subject-matter of the invention is a tire comprising asemi-finished article according to the invention.

The invention is illustrated by the following examples.

EXAMPLE 1—Preparation of an Elastomer Matrix According to the Invention1.1—Measurements and Tests Used—Experimental Techniques Used for thePre-Curing Characterization of the Polymers Obtained

(a) Determination of the Molar Mass Distribution by the Size ExclusionChromatography (Conventional SEC) Technique

Size exclusion chromatography or SEC makes it possible to separatemacromolecules in solution according to their size through columnsfilled with a porous gel. The macromolecules are separated according totheir hydrodynamic volume, the bulkiest being eluted first.

Without being an absolute method, SEC makes it possible to comprehendthe distribution of the molar masses of a polymer. The variousnumber-average molar masses (Mn) and weight-average molar masses (Mw)and the weight at the peak (Wp) can be determined from commercialstandards and the polydispersity index (PI=Mw/Mn) can be calculated viaa “Moore” calibration.

(1) Preparation of the Polymer:

There is no specific treatment of the polymer sample before analysis.The latter is simply dissolved in tetrahydrofuran at a concentration ofapproximately 1 g/l. The solution is then filtered through a filter witha porosity of 0.45 μm before injection.

(2) SEC Analysis:

The apparatus used is a “Waters Alliance” chromatograph. The elutionsolvent is tetrahydrofuran, the flow rate is 0.7 ml/min, the temperatureof the system is 35° C. and the analytical time is 90 min. A set of fourWaters columns in series, with commercial names “Styragel HMW7”,“Styragel HMW6E” and two “Styragel HT6E”, is used.

The volume of the solution of the polymer sample injected is 100 μl Thedetector is a “Waters 2410” differential refractometer and the softwarefor making use of the chromatographic data is the “Waters Empower”system.

The calculated average molar masses relate to a calibration curveproduced with polystyrene standards having known molar masses.

(b) Determination of the Percentage by Weight of a Population of PolymerChains by the High Resolution Size Exclusion Chromatography Technique(HR SEC Analysis)

(1) Preparation of the Polymer:

There is no specific treatment of the polymer sample before analysis.The latter is simply dissolved in tetrahydrofuran at a concentration ofapproximately 1 g/l. The solution is then filtered through a filter witha porosity of 0.45 μm before injection.

(2) HR SEC Analysis:

The apparatus used is a “Waters Alliance” chromatographic line. Theelution solvent is tetrahydrofuran, the flow rate is 0.2 ml/min, thetemperature of the system is 35° C. and the analytical time is 205 min.A set of 3 Shodex columns in series, with the commercial name “KF805”,is used.

The volume of the solution of the polymer sample injected is 50 μl. Thedetector is a “Waters 2410” differential refractometer and the softwarefor making use of the chromatographic data is the “Waters Empower”system.

The chromatogram obtained is composed of several distinct peaks. Apopulation of polymer chains corresponds to each peak.

The value of the refractometric signal at the baseline is zero.

The percentages by weight of each population are determined by the ratioof the value of the refractometric signal at the peak top for thepopulation concerned to the sum of the values of the refractometricsignal at the peak top for all the populations.

(c) For the Polymers and Rubber Compositions, the Mooney Viscosities ML(1+4) at 100° C. Are Measured According to Standard ASTM D-1646.

Use is made of an oscillating consistometer as described in StandardASTM D-1646. The Mooney plasticity measurement is carried out accordingto the following principle: the composition in the raw state (i.e.,before curing) is moulded in a cylindrical chamber heated to 100° C.After preheating for one minute, the rotor rotates within the testspecimen at 2 revolutions/minute and the working torque for maintainingthis movement is measured after rotating for 4 minutes. The Mooneyplasticity (ML 1+4) is expressed in “Mooney unit” (MU, with 1 MU=0.83N.m).

(d) The glass transition temperatures Tg of the polymers are measuredaccording to Standard ASTM D3418-03 using a differential scanningcalorimeter.

(e) Near-infrared spectroscopy (NIR) is used to quantitatively determinethe content by weight of styrene in the elastomer and its microstructure(relative distribution of the 1,2-vinyl, trans-1,4 and cis-1,4 butadieneunits). The principle of the method is based on the Beer-Lambert lawgeneralized for a multicomponent system. As the method is indirect, itinvolves a multivariate calibration [Vilmin, F., Dussap, C. and Coste,N., Applied Spectroscopy, 2006, 60, 619-29] carried out using standardelastomers having a composition determined by ¹³C NMR. The styrenecontent and the microstructure are then calculated from the NIR spectrumof an elastomer film having a thickness of approximately 730 μm. Thespectrum is acquired in transmission mode between 4000 and 6200 cm⁻¹with a resolution of 2 cm⁻¹ using a Bruker Tensor 37 Fourier-transformnear-infrared spectrometer equipped with an InGaAs detector cooled bythe Peltier effect.

(f) For the polymers, the intrinsic viscosity at 25° C. of a 0.1 g/dlsolution of polymer in toluene is measured starting from a solution ofdry polymer:

The intrinsic viscosity is determined by the measurement of the flowtime t of the polymer solution and of the flow time t_(o) of the toluenein a capillary tube.

The flow time of the toluene and the flow time of the 0.1 g/dl polymersolution are measured in a Ubbelohde tube (diameter of the capillary0.46 mm, capacity from 18 to 22 ml) placed in a bath thermostaticallycontrolled at 25±0.1° C.

The intrinsic viscosity is obtained by the following relationship:

$n_{inh} = {\frac{1}{C}{\ln \left\lbrack \frac{(t)}{\left( t_{O} \right)} \right\rbrack}}$

with:

C: concentration of the toluene solution of polymer in g/dl;

t: flow time of the toluene solution of polymer in seconds;

t_(o): flow time of the toluene in seconds;

η_(inh): intrinsic viscosity, expressed in dl/g.

(g) For the polymers, the cold flow: CF100(1+6), results from thefollowing measurement method:

It is a matter of measuring the weight of rubber extruded through acalibrated die over a given time (6 hours), under fixed conditions (at100° C.). The die has a diameter of 6.35 mm for a thickness of 0.5 mm.

The cold flow apparatus is a cylindrical cup pierced at the base.Approximately 40 g±4 g of rubber, preprepared in the form of a pellet(thickness of 2 cm and diameter of 52 mm), are placed in this device. Acalibrated piston weighing 1 kg (±5 g) is positioned on the rubberpellet. The assembly is subsequently placed in an oven thermallystabilized at 100° C.±0.5° C.

During the first hour in the oven, the measurement conditions are notstabilized. After one hour, the product which has extruded is thus cutoff and discarded.

The measurement subsequently lasts 6 hours±5 min, during which theproduct is left in the oven. At the end of the 6 hours, the extrudedproduct sample has to be recovered by cutting it flush with the surfaceof the base. The result of the test is the weight of rubber weighed.

1.2—Preparation of the Functionalized Elastomers According to theInvention and Control for the Elastomers (a) Preparation of a LivingPolyisoprene

The living polyisoprene is prepared batchwise in a reactor with areaction volume of 75 l, under nitrogen pressure, which reactor isequipped with a stirrer of turbine type. On the one hand,methylcyclohexane and isoprene according to the respective ratios byweight of 100/25 and, on the other hand, an amount of 10 000 μmol ofactive sec-butyllithium (s-BuLi) per 100 g of isoprene are introducedinto this reactor.

The temperature of the reactor is maintained at 45° C. and, after apolymerization time of 30 min, the conversion of monomers is 100%.

The concentration of active polyisoprenyllithium (PILi) is determined ona withdrawn sample by successive titrations with perchloric acid, wherethe activity of this living polyisoprene is calculated by differencebetween the total basicity and the residual basicity:

Assaying of the total basicity: approximately 1000 μmol of PILi(solution to be assayed) are introduced and 25 ml of acetic acid areintroduced into 50 ml of toluene. A few drops of an acetic solution ofcrystal violet are introduced. Assaying is carried out with perchloricacid (0.1M in acetic acid); the solution changes from purple to emeraldgreen at the equivalence point and then to yellow when the equivalencepoint has been exceeded.

Assaying of the residual basicity: approximately 1000 μmol oftrans-1,4-dibromo-2-butene are introduced. Approximately 1000 μmol ofPILi (solution to be assayed) are introduced into 50 ml of toluene.After 10 minutes, 25 ml of acetic acid are added and a few drops of anacetic solution of crystal violet are introduced. Assaying is carriedout with perchloric acid (0.1M in acetic acid); the solution changesfrom purple to emerald green at the equivalence point and then to yellowwhen the equivalence point has been exceeded.

Results: the concentration of the active polyisoprenyllithium iscalculated according to:

$t_{PILi} = {\frac{\left( {V_{T} - V_{R}} \right)}{V_{PILi}} \times t_{{HClO}\; 4}}$

where:

V_(T)=volume of HClO₄ necessary for the assaying of the total basicity(in ml)

V_(R)=volume of HClO₄ necessary for the assaying of the residualbasicity (in ml)

t_(HClO4)=concentration of the perchloric acid solution (in mol/l)

V_(PILi)=volume of the PILi solution used for the sampling (in ml)

t_(PILi)=concentration of the PILi solution analysed (in mol/l)

The concentration of the living polyisoprene is thus estimated at 0.0157mol/l. This lithiated polyisoprene is stored under nitrogen at atemperature of 10° C. No change in the content is observed duringstorage for several weeks under nitrogen pressure at this temperature.

The number-average molecular weight and the polydispersity index of theliving polyisoprene thus obtained, which are determined by conventionalSEC on a withdrawn sample stopped by one lithium equivalent of methanol,are 9307 g/mol and 1.07 respectively. The content of 3,4 linkages ofthis polyisoprene is 23.7%.

(b) Preparation of a Control Elastomer a (Denoted SBR A):Copolymerization of Butadiene and Styrene Initiated by a LivingPolyisoprene

The block copolymer other than the said living polyisoprene is preparedbatchwise in a reactor with a reaction volume of 75 l, under nitrogenpressure, which reactor is equipped with a stirrer of turbine type.Methylcyclohexane, butadiene and styrene are introduced into thisreactor according to respective proportions by weight of 100/10/6.6. 600parts per million (by weight) of tetrahydrofuran (THF), as agentpromoting vinyl bonds, are also added to this reactor.

An amount of 57 μmol of active n-butyllithium (n-BuLi) per 100 g ofsolution is introduced into the reactor in order to neutralize theprotic impurities which are contributed by the various constituentspresent in the reactor with the aim of limiting the formation of dead ordeactivated polyisoprene during the introduction of the livingpolyisoprene solution into the reactor.

545 μmol per 100 g of monomers of the living polyisoprene described insection 1-2) (a), representing the amount of active initiator forinitiating the polymerization, are introduced.

The temperature of the reactor is maintained at 50° C. and, after apolymerization time of 35 min, the conversion of monomers is 70%.

The intrinsic viscosity of the copolymer before functionalization,measured on a withdrawn sample stopped by one lithium equivalent ofmethanol, is 1.2 dl/g and the number-average molecular weight and thepolydispersity index of the same withdrawn sample, which are determinedby conventional SEC, are 106 424 g/mol and 1.07 respectively.

262 μmol/100 g of monomers of a coupling agent consisting ofdibutyldichlorotin are subsequently added to the same reactor. Thefunctionalization reaction is carried out at 50° C. After 20 min of thiscoupling reaction, the block copolymer thus functionalized is subjectedto an anti-oxidizing treatment using 0.8 phr of2,2′-methylenebis(4-methyl-6-(tert-butyl)phenol) and 0.2 phr ofN-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine. Any reaction isstopped by the addition of 2 lithium equivalents of methanol.

The copolymer thus treated is separated from its solution by a steamstripping operation and is then dried on a screw machine at 150° C. for15 sec, in order to obtain the functionalized copolymer A comprisingcontrol blocks.

The intrinsic viscosity of this copolymer A is 1.96 dl/g and its MLviscosity is 84.

The cold flow of this copolymer A is 3.3.

The SBR block of this copolymer A comprises 28.8% of styrene (by weight)and, for its butadiene part, 25.2% of vinyl units.

The glass transition temperature of this copolymer A is −50° C.

The number-average molecular weight and the polydispersity index of thiscopolymer A, which are determined by conventional SEC, are 166 200 g/moland 1.12 respectively.

The percentages by weight of each population present in the copolymer Aare determined by HR SEC. 2 predominant populations are distinguished: apopulation corresponding to 1 branch, with a weight at the peak ofapproximately 110 000 g/mol and a percentage by weight of approximately4%, and a population corresponding to 2 branches or to a coupledcopolymer, with a weight at the peak of approximately 210 000 g/mol anda percentage by weight of approximately 96%.

(c) Preparation of an Elastomer C According to the Invention (DenotedSBR C): Copolymerization of Butadiene and Styrene Initiated by a LivingPolyisoprene

The block copolymer other than the said living polyisoprene is preparedbatchwise in a reactor with a reaction volume of 75 l, under nitrogenpressure, which reactor is equipped with a stirrer of turbine type.Methylcyclohexane, butadiene and styrene are introduced into thisreactor according to respective proportions by weight of 100/10/6.6. 600parts per million (by weight) of tetrahydrofuran (THF), as agentpromoting vinyl bonds, are also added to this reactor.

An amount of 45 μmol of active n-butyllithium (n-BuLi) per 100 g ofsolution is introduced into the reactor in order to neutralize theprotic impurities which are contributed by the various constituentspresent in the reactor with the aim of limiting the formation of dead ordeactivated polyisoprene during the introduction of the livingpolyisoprene solution into the reactor.

550 μmol per 100 g of monomers of the solution of the livingpolyisoprene described in section 1-2) (a), representing the amount ofactive initiator for initiating the polymerization, are introduced.

The temperature of the reactor is maintained at 50° C. and, after apolymerization time of 35 min, the conversion of monomers is 70%.

The intrinsic viscosity of the copolymer before functionalization,measured on a withdrawn sample stopped by one lithium equivalent ofmethanol, is 1.22 dl/g and the number-average molecular weight and thepolydispersity index of the same withdrawn sample, which are determinedby conventional SEC, are 105 000 g/mol and 1.07 respectively.

Subsequently, 105.6 μmol/100 g of monomers of a coupling agentconsisting of dibutyldichlorotin and 82.5 μmol/100 g of monomers of astar-branching agent consisting of tetrachlorotin are added to the samereactor. The functionalization reactions are carried out at 50° C. After20 min of reaction, the block copolymer thus functionalized is subjectedto an anti-oxidizing treatment using 0.8 phr of2,2′-methylenebis(4-methyl-6-(tert-butyl)phenol) and 0.2 phr ofN-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine. Any reaction isstopped by the addition of 2 lithium equivalents of methanol.

The copolymer thus treated is separated from its solution by a steamstripping operation and is then dried on a screw machine at 150° C. for15 sec, in order to obtain the functionalized elastomer C according tothe invention.

The intrinsic viscosity of this copolymer C is 2.32 dl/g and its MLviscosity is 105.

The cold flow of this copolymer C is 0.2.

The SBR block of this copolymer C comprises 28.9% of styrene (by weight)and, for its butadiene part, 25.1% of vinyl units.

The glass transition temperature of this copolymer C is −48° C.

The number-average molecular weight and the polydispersity index of thiscopolymer C, which are determined by conventional SEC, are 227 000 g/moland 1.30 respectively.

The percentages by weight of each population present in the copolymer Caccording to the invention are determined by HR SEC. 3 predominantpopulations are distinguished: a population corresponding to 2 branches,block copolymer a) according to the invention, with a weight at the peakof approximately 220 000 g/mol and a percentage by weight ofapproximately 55%, a population corresponding to a star-branchedcopolymer having 4 branches, block copolymer b) according to theinvention, with a weight at the peak of approximately 400 000 g/mol anda percentage by weight of approximately 35%, and a populationcorresponding to 1 branch, with a weight at the peak of approximately115 000 g/mol and a percentage by weight of approximately 10%. Thelatter population can be regarded as the non-functional block copolymerc) according to the invention, taking into account the content of thecoupling and star-branching agents used for the functionalization.

(d) Preparation of an Elastomer B According to the Invention (DenotedSBR B): Mixture of the Copolymers A and C

The elastomer B according to the invention is prepared by mixing theelastomers A and C in the proportions ⅓ and ⅔ respectively. A solutionwith methylcyclohexane and the elastomers A and C in the proportions100/3.7/7.3 respectively is prepared.

Once all the elastomer has dissolved and the solution is homogeneous,the elastomer thus obtained is separated from its solution by drying inan oven at 60° C., under vacuum at 200 mmHg and under a nitrogenatmosphere. After 24 hours, the elastomer B according to the inventionis obtained.

The intrinsic viscosity of this copolymer B is 2.08 dl/g and its MLviscosity is 95.

The cold flow of this copolymer B is 0.7.

The SBR block of this copolymer B comprises 28.8% of styrene (by weight)and, for its butadiene part, 25.0% of vinyl units.

The glass transition temperature of this copolymer B is −47.8° C.

The number-average molecular weight and the polydispersity index of thiscopolymer B, which are determined by conventional SEC, are 213 400 g/moland 1.26 respectively.

The percentages by weight of each population present in the copolymer Baccording to the invention are determined by HR SEC. 3 predominantpopulations are distinguished: a population corresponding to 2 branches,block copolymer a) according to the invention, with a weight at the peakof approximately 225 000 g/mol and a percentage by weight ofapproximately 64%, a population corresponding to a star-branchedcopolymer having 4 branches, block copolymer b) according to theinvention, with a weight at the peak of approximately 410 000 g/mol anda percentage by weight of approximately 24%, and a populationcorresponding to 1 branch, with a weight at the peak of approximately115 000 g/mol and a percentage by weight of approximately 12%. Thelatter population can be regarded as the non-functional block copolymerc) according to the invention, taking into account the content of thecoupling and star-branching agents used for the functionalization.

Comparative Examples of Rubber Compositions

A) Measurements and Tests Used

a) the Mooney viscosity ML (large rotor) or MS (small rotor) (1+4) at100° C.: measured according to Standard ASTM: D-1646, entitled “Mooney”,

(b) the Shore A hardness: measurements carried out according to StandardDIN 53505,

(c) the tensile tests make it possible to determine the elasticitystresses and the properties at break. Unless otherwise indicated, theyare carried out in accordance with French Standard NF T 46-002 ofSeptember 1988. Processing the tensile recordings also makes it possibleto plot the curve of modulus as a function of the elongation, themodulus used here being the nominal (or apparent) secant modulusmeasured in first elongation, calculated by reducing to the initialcross-section of the test specimen. The nominal secant moduli (orapparent stresses, in MPa) are measured in first elongation, at 60°C.±2° C., at 10%, 100% and 300% elongation, respectively denoted MSA10,MSA100 and MSA300. The breaking stresses (BS) in MPa and the elongationsat break (EB) in % are measured at 60° C.±2° C. according to Standard NFT 46-002.

(d) the dynamic properties ΔG* and tan(δ)max are measured on a viscosityanalyser (Metravib VA4000) according to Standard ASTM D 5992-96. Theresponse of a sample of vulcanized composition (cylindrical testspecimen with a thickness of 2 mm and a cross-section of 79 mm²),subjected to a simple alternating sinusoidal shear stress, at afrequency of 10 Hz, under standard temperature conditions (23° C.)according to Standard ASTM D 1349-99 or, as the case may be, at adifferent temperature (60° C.), is recorded. A peak-to-peak strainamplitude sweep is carried out from 0.1% to 50% (outward cycle) and thenfrom 50% to 0.1% (return cycle). The results made use of are the complexdynamic shear modulus (G*) and the loss factor tan(δ). The maximum valueof tan(δ) observed (tan(δ)max) and the difference in complex modulus(ΔG*) between the values at 0.1% and 50% strain (Payne effect) are shownfor the return cycle.

(e) for the polymers, the cold flow CF100(1+6) results from thefollowing measurement method: it is a matter of measuring the weight ofgum extruded through a calibrated die over a given time (6 hours), underfixed conditions (at 100° C.). The die has a diameter of 6.35 mm for athickness of 0.5 mm.

The cold flow apparatus is a cylindrical cup pierced at the base.Approximately 40 g±4 g of rubber, preprepared in the form of a pellet(thickness of 2 cm and diameter of 52 mm), are placed in this device. Acalibrated piston weighing 1 kg (±5 g) is positioned on the rubberpellet. The assembly is subsequently placed in an oven thermallystabilized at 100° C.±0.5° C.

During the first hour in the oven, the measurement conditions are notstabilized. After one hour, the product which has extruded is thus cutoff and discarded. The measurement subsequently lasts 6 hours±5 min,during which the product is left in the oven. At the end of the 6 hours,the extruded product sample has to be recovered by cutting it flush withthe surface of the base. The result of the test is the weight of rubberweighed.

B) Example

In this example, the three elastomers SBR A, SBR B and SBR C were usedfor the preparation of rubber compositions A, B and C, each comprisingcarbon black as reinforcing filler.

Each of these compositions A, B and C exhibits the following formulation(expressed in phr: parts per hundred parts of elastomer):

Elastomer 100 N234 54 Paraffin 1 Antioxidant (1) 4 Stearic acid 1.5 ZnO3 Sulphur 1.3 Accelerator (2) 1.3 (1)N-(1,3-dimethylbutyl)-N-phenyl-para-phenylenediamine (6-PPD) (2)N-cyclohexyl-2-benzothiazolesulphenamide (CBS)

Each of the following compositions is produced, in a first step, bythermomechanical working and then, in a second finishing step, bymechanical working.

The elastomer, the black, the paraffin, the antioxidant, the stearicacid and the zinc monoxide are successively introduced into a laboratoryinternal mixer of “Banbury” type which has a capacity of 400 cm³, whichis 75% filled and which has an initial temperature of approximately 70°C. The stage of thermomechanical working is carried out for from 5 to 6minutes, up to a maximum dropping temperature of approximately 160° C.The abovementioned first step of thermomechanical working is thuscarried out, it being specified that the mean speed of the blades duringthis first step is 70 revolutions/min.

The mixture thus obtained is recovered and cooled and then, in anexternal mixer (homofinisher), the sulphur and the accelerator are addedat 30° C., the combined mixture being further mixed for a time of 3 to 4minutes (abovementioned second step of mechanical working).

The compositions thus obtained are subsequently calendered, either inthe form of plaques (with a thickness ranging from 2 to 3 mm) or of thinsheets of rubber, for the measurement of their physical or mechanicalproperties. The vulcanization is carried out at 150° C. for 15 minutes.

The properties of these three compositions are compared with oneanother, both in the non-vulcanized state and in the vulcanized state.The results are given in the following table:

Composition A B C Elastomer SBR A SBR B SBR C Wp Weight at the peak of110 000 115 000 115 000 the elastomer branch [AB] ML(1 + 4) 100° C. 8495 105 (elastomer) CF100(1 + 6) (elastomer) 3.3 0.7 0.2 Properties inthe non-vulcanized state: MS(1 + 4) 100° C. 58 61 64 (mixture)Properties in the vulcanized state: Shore A 67 67 68 MSA10 5.85 5.835.93 MSA100 2.70 2.66 2.69 MSA300 5.35 5.18 5.27 MSA300/MSA100 1.98 1.951.96 Losses 60° C. (%) 23 23 23 Tension: BS (MPa) 21 19 21 EB (%) 356338 364 Dynamic properties as a function of the strain: ΔG* (MPa) at 60°C. 1.37 1.51 1.42 Tanδ_(max) at 60° C. 0.146 0.153 0.146

It should be noted that the functionalized diene elastomers B and Caccording to the invention exhibit lower cold flow values than that ofthe control functionalized diene elastomer A. The functionalized dieneelastomers B and C according to the invention thus exhibit an improvedcold flow with respect to the control functionalized diene elastomer A.

Furthermore, the compositions B and C according to the invention, basedon the said functionalized diene elastomers B and C, exhibit a “mixture”Mooney value similar to that of the control composition A based on thefunctionalized diene elastomer A, taking into account the difference inbranch length observed. The compositions B and C according to theinvention and the control composition A thus exhibit an equivalentprocessability in the non-vulcanized state.

As regards the properties in the vulcanized state, the compositions Band C according to the invention and the control composition A exhibitequivalent properties and in particular similar hysteresis properties.

In other words, the compositions B and C according to the invention,based on the said functionalized diene elastomers B and C, exhibitrubber properties in the non-crosslinked state and in the crosslinkedstate which are equivalent, with respect to those of the composition Abased on the control functionalized diene elastomer A, the cold flowfurthermore being improved for the functionalized diene elastomers B andC according to the invention, with respect to the control functionalizeddiene elastomer A.

1. Functionalized diene elastomer composed: a) of a block copolymerfunctionalized, at the chain end or in the middle of the chain, by a tinfunctional group and corresponding to the following formula:[A-B_(n)XB-A]_(m) where n and m are integers of greater than or equalto 0 and such that n+m=1 or 2, b) of a block copolymer star-branched bytin and corresponding to the following formula:[A-B_(o)YB-A]_(p) where o and p are integers of greater than or equalto 0 and such that o+p≧3 and o+p≦6, c) of a content of less than 15% byweight, with respect to the total weight of the functionalized dieneelastomer, of a non-tin-functional block copolymer corresponding to thefollowing formula:[A-B] where: all the A blocks are composed of a polyisoprene or all theA blocks are composed of a polybutadiene, the B blocks are composed of adiene elastomer, the molar content of units resulting from conjugateddienes of which is greater than 15%, the B blocks being identical to oneanother, X and Y independently represent a tin-comprising group, thenumber-average molecular weight Mn1 of each A block varies from 2 500 to20 000 g/mol, the number-average molecular weight Mn2 of each B blockvaries from 80 000 to 350 000 g/mol, the content of 1,2 linkages in eachA block is between 1 and 20% in the case where A is a polybutadieneblock, the content of 3,4 linkages in each A block is between 1 and 25%in the case where A is a polyisoprene block, the A-B copolymer exhibitsa monomodal distribution of molecular weights before optionalfunctionalization or optional star-branching and a polydispersity indexbefore optional functionalization or optional star-branching of lessthan or equal to 1.3.
 2. Elastomer according to claim 1, wherein itcomprises from 5% to 45% by weight, with respect to the total weight ofthe functionalized diene elastomer, of the said block copolymerstar-branched by tin b).
 3. Elastomer according to claim 1, wherein itcomprises a content strictly of greater than 0% by weight and of lessthan 10% by weight, with respect to the total weight of thefunctionalized diene elastomer, of the said non-tin-functional blockcopolymer c).
 4. Elastomer according to claim 1, wherein the ratio ofthe number-average molecular weight Mn1 of each end polybutadiene orpolyisoprene block A to the number-average molecular weight Mn2 of eachof the B blocks varies from 5 to 20%.
 5. Elastomer according to claim 1,wherein the functionalization of the copolymer a) is obtained with amonohalotin functionalization agent or a dihalotin coupling agent. 6.Elastomer according to claim 1, wherein the star-branching is obtainedwith a tri- or tetrahalotin star-branching agent.
 7. Elastomer accordingto claim 1, wherein the B block or blocks are chosen from copolymers ofstyrene and butadiene, copolymers of styrene and isoprene, copolymers ofbutadiene and isoprene, styrene/butadiene/isoprene terpolymers,polyisoprene when the neighbouring A block is a polybutadiene andpolybutadiene when the neighbouring A block is a polyisoprene. 8.Elastomer according to claim 1, wherein the block copolymer a) is acopolymer functionalized by a tin functional group in the middle of thechain and the block copolymer b) is a copolymer star-branched by tinhaving 4 branches.
 9. Elastomer according to claim 1, wherein A ispolyisoprene and B is a copolymer of styrene and butadiene, the blockcopolymer a) being functionalized by the functionalization agentBu₂SnCl₂ and the block copolymer b) being star-branched by thestar-branching agent SnCl₄.
 10. Elastomer according to claim 1, whereinA is polybutadiene and B is a copolymer of styrene and butadiene, theblock copolymer a) being functionalized by the functionalization agentBu₂SnCl₂ and the block copolymer b) being star-branched by thestar-branching agent SnCl₄.
 11. Reinforced rubber composition based onat least one reinforcing filler and on an elastomer matrix, wherein theelastomer matrix comprises at least one functionalized diene elastomeras defined in claim
 1. 12. Composition according to claim 11, whereinthe elastomer matrix also comprises at least one diene elastomer otherthan the said functionalized diene elastomer.
 13. Composition accordingto claim 11, wherein the composition further comprises a chemicalcrosslinking agent.
 14. Semi-finished article made of rubber for a tire,wherein it comprises a crosslinkable or crosslinked rubber compositionas defined in claim
 11. 15. Semi-finished article according to claim 14,wherein the said article is a tread.
 16. Tire, wherein it comprises asemi-finished article as defined in claim
 14. 17. Elastomer according toclaim 2, wherein it comprises from 10% to 30% by weight of said blockcopolymer star-branched by tin b).
 18. Elastomer according to claim 3,wherein it comprises a content of less than 5% by weight of the saidnon-tin-functional block copolymer c).